Atmospheric icing occurs on aircraft when flying through supercooled water. When aircraft contact such supercooled water, phase transition of the water to ice occurs. Typically, icing occurs between 0° C. and −20° C. At temperatures higher than this range, water is not supercooled. At temperatures lower than this range, water usually exists in the atmosphere as ice.
There are two kinds of ice that commonly form on aircraft, glaze ice and rime ice. Glaze ice forms at warmer outside air temperatures just below 0° C. Glaze ice is a hard ice that can cause engine damage if shed in large pieces. It is also dense and heavy and can affect aircraft lift. Rime ice occurs at lower temperatures and is porous and brittle, with less chance of engine damage if shed. Rime ice is typically less dense and does not commonly affect aircraft lift. In addition, there is “Glime ice” that is a mix of rime ice and glaze ice, formed during transition temperatures between rime ice and glaze ice.
Icing conditions require atmospheric water content, generally a function of ground temperature, altitude and the type of cloud formed through which the aircraft is flying. FIG. 1 herein shows a bell curve of icing conditions as a function of temperature, altitude and droplet diameter and atmospheric water content.
In-flight icing forms when an aircraft flies through a cloud of super-cooled precipitation. As the aircraft flies, it causes the portion of the air that it encounters to move around it rapidly. Water droplets resident in that air cannot move rapidly enough, due to their mass, to avoid the aircraft and instead strike or impinge the aircraft surfaces. When such water droplets are supercooled, they change phases to solid when they strike or impinge the aircraft surfaces. Ice therefore forms on the leading or forward-facing edges of the wings, tail, antennas, windshield, radome, engine inlet, and so forth.
A dangerous way that ice acts on an aircraft is through its effect on the aerodynamics, which results in degraded performance and control. Small amounts of ice or frost add roughness to the airplane surfaces. The roughness increases the friction of the air over the surface; this is called skin friction. Large accretions can drastically alter the shape of the wing. Then, in addition to skin friction, flow separation results in a further reduction in aerodynamic performance of the aircraft. Large accretions can also cause increased weight of the aircraft, further degrading flight performance.
Aircraft control can be seriously affected by ice accretion. Ice accretion on the tail can lead to reduced elevator effectiveness, reducing the longitudinal control (nose up and down) of the aircraft. In some situations, the tail can stall or lose lift prematurely, resulting in the aircraft pitching nose down. Similarly, ice on the wing ahead of the aileron can result in roll upset. Both tail stall and roll upset are thought to be the cause of recent aircraft icing accidents.
Large aircraft and many light aircraft are equipped with in-flight ice protection systems to reduce the effect of ice. Ice protection systems are classified as de-ice or anti-ice systems. De-ice systems allow some ice to accrete, and then they periodically remove the ice. Anti-ice systems prevent ice from forming either by heating the surface above 0° C. (32° F.) or through the use of freezing-point depressants.
When aircraft encounter supercooled atmospheric water, and ice forms on wings and engines, aircraft can experience loss of lift due to weight gain and can be subject to changes in aerodynamics. When ice that has formed on an engine intake manifold or cowling fractures and breaks free, it can enter the engine and cause catastrophic mechanical damage.
If icing conditions are known to exist, or if icing begins to occur, pilots can sometimes take navigational action to avoid serious consequences. Therefore, in order for pilots to be warned of icing as soon as possible, there is a need for a system that (1) detects conditions favorable to icing, (2) detects the presence of ice, and (3) measures the rate of icing accretion. The present disclosure addresses the foregoing needs.
For aircraft application, in-flight ice detection is limited to in situ sensors, such as the mechanical Ice Detector by Rosemount Aerospace which is mounted on an aircraft surface to sense a collection of ice on a vibrating element. This sensor saturates quickly and requires heating to return to its initial state. It is also intrusive and therefore not suitable for certain aircraft environments and surfaces. A ground-based ice detection system, known as the IceHawk system by Sensor Systems Division of Goodrich Corporation, detects accreted ice on the surface of an aircraft visually by laser polarization scanning techniques. Neither system can predict ice accretion prior to the formation on the aircraft surface or before the aircraft enters an icing region of airspace during flight.
Similar temperature and relative humidity measurements are performed by interrogating atmospheric nitrogen gas and water vapor with Raman LIDAR devices. However, such devices require expensive hardware and are configured for measurements that, while stand-off, are 400 m to 3 km away from the Raman systems. Thus, such devices are not suitable for direct transfer to an engine inlet environment. Alternately, standard Raman spectrometers are used to measure types of condensed phase materials, but are not used to measure gas phase materials due to poor sensitivity of such spectrometers.
Accordingly, it is desirable to have a warning system small enough in size to be mountable on-board an aircraft and powered thereby and which has the capability of in-flight monitoring the aircraft for conditions likely to cause ice accretion on the surface of the aircraft and warn the pilot and crew of such an impending condition in sufficient time to change the heading of the aircraft. Therefore, there is a need in the art for such a system to provide sufficient flexibility that it can be deployed in a plurality of sensor response and physical configurations so that it can be used in a wide variety of applications for aircraft and other structures. There is a further need in the art for such a warning system to contain a robust set of features that enable it to maintain its sensitivity and calibration under harsh temperature, pressure and vibration conditions, to return its sensitivity after an icing event has occurred to a state that can indicate new icing events, to provide information regarding the type of ice that forms on the aircraft, and to operate effectively under a wide variety of flight conditions. The present disclosure is made to address the foregoing needs.